Dehumidification of moist gas mixtures is necessary in a multitude of technical fields. For instance ventilation and air conditioning of buildings or vehicles generally necessitates not only cooling but also dehumidification of the air since the air to be cooled is often so humid that during cooling to the desired temperature the temperature falls below the dew point temperature. Hence in conventional air conditioning systems dehumidification of the air accounts for a large part of the electricity consumption.
The electricity consumption of air conditioning systems for buildings is reduced by dehumidifying the air by adsorption or absorption of water with a drying medium and subsequently regenerating the water-laden drying medium by heating to a temperature at which the water is desorbed again. Compared to adsorption on a solid absorbent, the advantage of absorption in a liquid absorption medium is that air dehumidification can be carried out with reduced equipment complexity and with less drying medium and that regeneration of the water-laden drying medium using solar heat is easier to carry out.
A further technical field where dehumidifying of moist gas mixtures is employed is the field of absorption chillers (synonymous with “absorption heat pump”, principle described in WO 2014/079675 A1). Here, the damp gas mixture is formed during evaporation of water under low pressure. The water vapour thus formed needs to be removed from the moist gas mixture so that said mixture may then be returned to the water evaporation to pass through a new cycle. Here too, absorption in a liquid absorption medium is favoured over adsorption on a solid adsorption medium.
To absorb moisture from ionic liquids the prior art suggests a series of ionic liquids. Thus, DE 10 2010 004 779 A1 mentions 1-ethyl-3-methylimidazolium ethylsulfate, 1-ethyl-3-methylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium diethylphosphate, 1-ethyl-3-methylimidazolium methylsulfonate, 1-butyl-3-methylimidazolium bistrifluoromethanesulfonimide, 1-butyl-3-ethylimidazolium chloride for example.
CN 102335545 A describes a series of ionic liquids based on alkyl phosphates, namely inter alia 1,3-dimethyimidazolium dimethylphosphate, 1-ethyl-3-methylimidazolium dimethylphosphate and 1-butyl-3-methylimidazolium dimethylphosphate.
Y. Luo et al., Appl. Thermal Eng. 31 (2011) 2772-2777 proposes using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate in place of an aqueous solution of lithium bromide for air dehumidification. However, this ionic liquid has the disadvantage of only poor absorption capability.
These and further ionic liquids are also discussed by Krannich et al., J. Chem. Eng. Data 61 (2016), 1162-1176.
Y. Luo et al., Solar Energy 86 (2012) 2718-2724 proposes the ionic liquid 1,3-dimethyimidazolium acetate as an alternative to 1-ethyl-3-methylimidazolium tetrafluoroborate for air dehumidification. The acetates are also discussed by M. Kanakubo et al., J. Mol. Liq. 217 (2016) 112-119.
US 2011/0247494 A1 proposes a further ionic liquid for this purpose, namely 1-ethyl-3-methylimidazolium acetate.
The application of these ionic liquids in absorption chillers is also discussed in WO 2013/050242 A1.
One important criterion for choosing an ionic liquid as absorption medium is the viscosity thereof. It is thus desirable to employ an absorption medium having the lowest possible viscosity since easier flowability of the absorption medium markedly simplifies the use thereof in air dehumidification. In an absorption chiller, in which the absorption medium needs to pumped, a more flowable absorption medium can be transported with reduced energy consumption which markedly enhances the economy of such a chiller. In addition, a lower viscosity also results in fewer outages caused for instance by cavity blockages.
Such blockages also have a further cause: It was observed that many ionic liquids described in the literature show a propensity to precipitate on contact with CO2, thus forming solid deposits. However, the presence of CO2 in the gas streams for dehumidification is in many cases unavoidable and in specific applications, for example dehumidification of natural gas, the proportion of CO2 in the gas stream for dehumidification is very high.
It is accordingly an object of the present inventions to provide an absorption medium which does not exhibit the abovementioned problems, i.e. has the lowest possible viscosity and a low propensity to precipitate on contact with CO2.
An absorption medium which achieves this object has now surprisingly been found.